Why does my blood sugar rise during exercise?

Man and Woman doing exercise

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Blood sugar regulation is a complicated process. On one hand, we know that maintaining stable blood glucose levels is a great way to improve metabolic health and reduce the likelihood of diabetes and cardiovascular disease. On the other hand, exercise can spike our blood sugar. If you are confused by this contradiction then you are not alone. Many people who are monitoring their blood sugar levels can become confused when they see a rise in glucose while performing an activity that is supposed to improve their health. We want to explain the reasons behind the workout-induced glucose rise in order to provide comfort that exercise is not harmful to our health. In fact, exercise is important for almost every aspect of our health, including blood sugar regulation. We hope to use this article to illuminate the most common reasons why blood sugar increases during exercise in order to provide comfort that this rise is not only normal but also helpful to fuel your workout. The benefits of exercise far outweigh the short-term increase in blood sugar induced by your physical activity.

1.       Lactic acid 

lactic acid and glucose for exercise

Whenever we perform physical activity, our body works incredibly hard to help us adapt and perform that exercise to the best of our ability. One way in which our physiology tries to boost our workout is by creating our own fuel. During the course of high-intensity exercise when oxygen levels are low (like interval training, weight lifting, or sprinting) lactic acid can collect in the blood. Our body can internally create glucose molecules by converting this lactic acid into glucose. As the glucose is created it is circulating through our bloodstream, raising our blood sugar. The good news is, this glucose is highly functional. It is transported to fuel our working muscles. This process is one way in which we can continue to power high-intensity exercise and keep up with the demands of our muscles (for a time).

2.       Adrenaline 

adrenaline and blood sugar

Adrenaline is a hormone that is commonly associated with extreme sports and daredevil activities. However, heavy weightlifting, sprints, and competitive sports also boost our adrenaline in order to provide us with enough energy to successfully perform high-intensity or explosive movements. Adrenaline tells the liver to release glycogen into the bloodstream in the form of glucose. The intent is to provide your muscles with the fuel they need to perform strenuous exercise. However, this mechanism is not perfect and sometimes it can overshoot. High-intensity exercise can cause the body to overreact and release more hormones (and thus blood sugar) than is needed.

3.       Fasted exercise 

fasted exercise and blood sugar

As if the interplay between blood sugar and exercise were not already overwhelming, we are about to add another counterintuitive element into the mix. Exercising on an empty stomach can spike your blood sugar. Yes, not eating anything and working out can raise blood sugar. This is especially true in the morning due to an effect called the “dawn phenomenon.”  The dawn phenomenon often occurs between 4 am and 8 am. During this time your liver can release glycogen (a stored form of glucose) as well as other hormones. The reason behind this glucose increase is to provide the brain with the fuel it needs to function. Thus, for those early risers who like to hit the gym without breakfast before work, the dawn phenomenon may be a reason for an increase in glucose levels.

4.       Food you eat before exercise 

blood sugar and exercise

Consuming a high volume of carbohydrates before a workout can also cause blood sugar to spike at the beginning of exercise. This is especially true if highly processed and sugary foods like sweet drinks, candy, or baked goods are consumed. The high concentration of exogenous sugar coupled with the additional glucose released as a result of hormones may produce a spike.

Putting it all together

For those of us working to control our blood sugar and optimize metabolic health, a rise in blood sugar during exercise without understanding why it is happening can deter us from working out. It is for this reason that we want to help reassure active individuals that exercise is a great way to improve metabolic health, even if you experience a rise in glucose during a workout. The body needs this blood sugar. When we use our muscles repeatedly throughout a workout, we are working to utilize that glucose as fuel and power us to new personal records. Exercise also increases insulin sensitivity. This is why blood sugar often comes down as we continue to exercise (especially for aerobic/cardiovascular workouts). In addition to our metabolic health, the benefits of exercise are widespread with regular physical activity reducing the risk of cardiovascular disease, lower blood pressure, and decrease the likelihood of obesity. As we work to better understand our blood sugar levels, don’t hesitate to hit the gym, go for a run, or enjoy your favorite type of physical activity.


Author: Dr. Colleen Gulick